Storage system and replication creation method thereof

ABSTRACT

In a storage system having a plurality of control units each connected with a plurality of disk units, it is provided that a replication is created in the volume of the disk units connected to different control units. The replication creation unit of a given control unit creates a replication in the volume of the disk unit connected to other control units in such a manner that the original volume information, the replication volume information in the control unit and information on the other control units are registered as volume pair information. Based on this volume pair information, a replication creation request is transmitted to the other control units.

This application is a continuation of U.S. patent application Ser. No.10/766,823, filed Jan. 30, 2004, now allowed, which is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a storage system comprising a pluralityof control units each connected with a plurality of disk units.

In recent years, demand has risen to shorten the time required for theprocess (hereinafter referred to as the backup process) to replicate thedata stored in a given storage unit to another storage unit held by abusiness organization. On the background of this demand is the increasein the information amount held by each organization, and the timeconsumed for backup is ever on the increase, while the increase in thebusiness hours of each organization has decreased the relative length oftime that can be assigned to the backup process.

A “snapshot” has been proposed in JP-A-7-210439 and JP-A-2001-318833 asa technique for backing up the data stored in a storage device withoutsuspending the routine work of organizations. The snapshot is defined asa function of producing a copy of a storage area of a storage device ata specific time point without the intermediary of the computer connectedwith the storage device. Taking advantage of the function, the user usesthe original storage area for business, and the data stored in the copystorage area for backup.

SUMMARY OF THE INVENTION

A cluster configuration storage system is conceived as a technique forincreasing the salability of a storage device connected to the network.The cluster configuration storage system is so configured that eachstorage system includes a plurality of clusters each constituting aconventional storage system such as a disk array unit.

No past reference is available which indicates a snapshot in theconventional cluster configuration storage system. In the case where thecluster configuration storage system and the conventional snapshottechnique are simply combined with each other, the storage area can becopied only in one cluster.

Unless a storage area can be copied between different clusters, however,a single cluster configuration storage system undesirably comes to haveboth a storage area where data can be copied and a storage area wheredata cannot be copied, thereby adversely affecting the scalabilityoriginally intended for by the cluster configuration storage system.

In the case where a copy of a logical volume (hereinafter referred tosimply as the volume) is produced between clusters of the clusterconfiguration storage system, i.e. in the case where an original volumeand a copy volume are existent in different clusters, the clusterassociated with the original volume (hereinafter sometimes referred toas the primary volume) cannot access a common memory in the clusterassociated with the copy volume (hereinafter sometimes referred to asthe secondary cluster), and therefore cannot recognize the load on thesecondary cluster. The user thus has no choice but to select a copyvolume in the same cluster as the original cluster, and therefore theuser operability is inconveniently changed due to a different systemconfiguration from the prior art.

Accordingly, an object of this invention is to provide a storage systemcomprising a plurality of control units each connected with a pluralityof disk units, wherein a copy of a storage area can be produced withoutbeing conscious of the difference of a control unit even in the casewhere a replication of a volume is created in the disk units connectedto different control units as well as in the disk units connected to thesame control unit.

According to this invention, there is provided a storage systemcomprising a plurality of control units each having a plurality of diskunits,

wherein each of the control units includes a replication creation unitfor creating a replication of the volume data in the disk unitsconnected to the control unit, and pair information having theinformation on the original volume and the information on thereplication volume,

wherein the replication creation unit of a first one of a plurality ofthe control units, when creating a replication in a volume in a diskunit connected to the same control unit, registers the information onthe original volume and the information on the replication volume as thepair information and thus creates a replication in the volume in thedisk unit connected to the same control unit based on the pairinformation, while when creating a replication in a volume in a diskunit connected to a second control unit, the replication creation unitregisters the information on the original volume, the information on thereplication volume of the first control unit and the information on thesecond control unit as the pair information, and

wherein a request to create a replication is transmitted to the secondcontrol unit based on the pair information thereby to create areplication.

In the storage system comprising a plurality of control units eachconnected with a plurality of disk units, therefore, a copy of a storagearea can be produced without being conscious of the difference of acontrol unit in the case where a replication is created in the volumesin the disk units connected to different control units as well as in thevolumes in the disk units connected to the same control unit.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a computer systemaccording to an embodiment of the invention.

FIG. 2 is a diagram showing a configuration of a memory according to anembodiment of the invention.

FIG. 3 is a diagram showing a configuration of a user input/outputapparatus (a management server, for example) according to an embodimentof the invention.

FIG. 4 is a diagram showing an example of a volume pair informationtable.

FIG. 5 is an example of a flowchart showing an outline of the processfor creating a replication.

FIG. 6 is an example of a flowchart showing a replication creatingmethod in the same storage control unit.

FIG. 7 is a diagram showing the process of a replication creating methodin different storage control units according to an embodiment of theinvention.

FIG. 8 a flowchart showing a method of creating a replication in adifferent storage control unit.

FIG. 9 is an example of a flowchart showing the process executed in thecase where a write request is issued during the initial copy operation.

FIG. 10 is a diagram showing an example of a volume information table.

FIG. 11 is a diagram showing an example of a difference bit map.

FIG. 12 is an example of a flowchart showing the split process.

FIG. 13 is an example of a flowchart showing the process of receiving awrite request to the primary volume and the process of receiving a writerequest to the secondary volume in fast split mode.

FIG. 14 is a diagram showing an example of a job priority informationbit map.

FIG. 15 is an example of a flowchart showing the resynchronizationprocess.

FIG. 16 is an example of a flowchart of the job priority informationregistration process and the read/write process based on the jobpriority information.

FIG. 17 is an example of a schedule-type flowchart of a low job priorityqueue.

FIG. 18 is a diagram for explaining the user operation for realizing apair between clusters as a pair in the same cluster.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an example of a computer system including a clusterconfiguration storage system according to an embodiment of theinvention.

A first storage system 70A is connected to hosts 10, 11 through anetwork 60. A user input/output apparatus 80 is connected to the firststorage system 70A through a management network 62, and a second storagesystem 70B to the first storage system 70A through a network 61. Theshown case represents a configuration connected with the second storagesystem 70B. Nevertheless, the second storage system 70B may not beconnected.

The storage system 70A has a protocol converting adaptor 40. Theprotocol converting adaptor 40 is a channel connecting portionindependent of a storage control unit 20, and handles a protocol basedon LAN (local area network), public network, dedicated line or ESCON(enterprise systems connection). A plurality of the protocol convertingadaptors 40 and a plurality of the storage control units 20 areconnected to each other through a network 63.

The protocol converting adaptor 40, upon receipt of an input/outputcommand from the hosts 10, 11, analyzes the command and converts theprotocol thereof. Then, determining a particular storage control unit 20controlling the LU (logical unit) storing the data requested by thecommand on the one hand and whether the LU is managed by the storagesystem 70B on the other, the adaptor 40 sends the command to the storagecontrol unit 20 thus determined. Which of the storage control units 20manages the LU is determined by accessing the configuration informationtable 510 stored on the memory in the processor 50 connected by anetwork 63.

The user input/output apparatus 80 recognizes the first storage system70A through the network 63. As an alternative, the user input/outputapparatus 80 is connected directly with the first storage system 70A bya dedicated line.

The storage control unit 20 includes a CPU 21, a memory 22, a cachememory 23 for temporarily storing the input/output data from the hosts10, 11, a hub 24 constituting a connecting portion with the network 63,and a storage I/F 25 for controlling the transmission and receipt of thedata to and from the storage units 31. These component parts areconnected with each other through an internal bus.

FIG. 2 is a diagram showing a configuration of the memory 22 accordingto an embodiment of the invention. As shown in FIG. 2, various programsexecuted by the CPU 21 are stored in the memory 22. Specifically, theprograms include a RAID (redundant array of inexpensive disks) controlprogram 200 for controlling the operation of the storage system 70, anda management agent 210 for managing the configuration of the storagesystem 70. The memory 22 also has stored therein various managementinformation. Specifically, the management information includes a volumepair information table 220 for recording the information on the originalvolume (replication source) and the copy volume (replication volume), avolume information table 230, a job priority information bit map 250, adifference bit map 240, a job queue 260 and a configuration informationtable (not shown) whereby the storage system 70B provides the storagesystem 70A with its own LU as a LU of the storage system 70A. Thecomponent parts 220 to 250 and the configuration information table maybe included in the processor 50.

The RAID control program 200 has a portion (not shown) from which acommand is issued to the storage units 31. The RAID control program 200has therein subprograms including a replication creation program 201 forcreating a replication (copy) of the data in the storage system 70 and ajob priority set program 201. The RAID control program 200, thereplication creation program 201 and the job priority set program 202are executed by the CPU 21 (FIG. 1) thereby to constitute a RAID controlunit, a replication creating unit and a job priority set unit of thestorage control unit 20. The RAID control program 200, the replicationcreation program 201 and the job priority set program 202 may partly orwholly be configured of hardware. The data is replicated eithersynchronously (a host notified of the completion of the data copy afterthe completion) or asynchronously (a host notified of the completion ofthe data copy before the completion). According to this embodiment,however, no distinction is made between the two variations.

The management agent 210 is a program for receiving an input from theuser input/output apparatus 80 and setting or outputting the informationto the user input/output apparatus 80.

FIG. 3 is a diagram showing a configuration of the user input/outputapparatus 80 according to an embodiment of the invention. The userinput/output apparatus 80 includes a CPU 81, a primary storage unit 82,an input unit (keyboard unit, etc.) 83, an output unit (display unit,etc.) 84, a management I/F 85 for connecting to the network 62 and astorage unit 86. These component parts are interconnected by an internalbus. The storage unit 86 has stored therein a job priority set program202 executed by the CPU 81.

The host 10 is a personal computer, a work station or a general-purposecomputer, and has a HBA (host bus adaptor) (not shown) constituting a FCinterface for external connection. HBA is also assigned WWN (world widename).

FIG. 4 is a diagram showing an example of the volume pair informationtable 220. This table contains information for managing the volume pairs(hereinafter sometimes referred to simply as the pairs) holding thereplicated data in the storage system 70, and includes a pair number221, primary volume information 222, secondary volume information 223 to225 and a pair status 226. Especially, the information on a pair locatedin the same storage control units 20A and the information on a pairhaving an original volume in the storage control unit 20A are stored inthe memory 22 of the storage control unit 20A.

The pair number 221 indicates an identifier arbitrarily assigned to eachpair.

The primary volume information 222 is the volume number assigned to theprimary volume constituting an original volume (original) of each pairassigned an identifier.

The secondary volume information 223 to 225 are the information on thesecondary volume constituting the replication (copy) volume of each pairassigned an identifier. The secondary volume number of a pair in thesame storage control unit is registered in column 223. As for a pairbetween different storage control units, on the other hand, a volumenumber virtualized in the same storage control unit is registered incolumn 223. The information on the storage control unit of the secondaryvolume for actually storing the data, such as the storage control unitnumber, is registered in column 224, and the volume number in column225. The volume number virtualized in the same storage control unit,though a volume number in the particular storage control unit, is avirtual volume number with which no volume is created for the disk unitconnected to the particular storage control unit.

The pair status 226 indicates the current state of a pair. The pairstatus includes a state (hereinafter referred to as a “paired state”) inwhich the data stored in the volumes of a pair are synchronized andcoincident with each other, and a state (hereinafter referred to as a“split state”) in which the data of a pair are not synchronized witheach other.

In the storage system 70A, a pair in paired state is changed to a pairin split state at an arbitrary time. At the same time, the data whichthe primary volume of the pair has at an arbitrary time is held in thesecondary volume (this process is called “to take a snapshot”). Afterthat, the host 10 reads the data from the secondary volume and writes itin another storage unit (such as a tape unit). Thus, the data stored inthe pair at the time when the snapshot is taken can be backed up. As analternative, the secondary volume after the snapshot is taken may beheld as a backup data.

FIG. 10 is a diagram showing an example of the volume information table230. This table is for registering the information to manage the volumescontrolled by the storage control unit 20A and storing the informationin the memory of the storage control unit 20A.

The volume number 231 is an identifier assigned to each volume. Thethree volume numbers 0 represents a case in which three pairs are setfor the same volume number 0. Generally, one or a plurality of pairs canbe set for each volume.

The column “primary/secondary” 232 indicates whether a given volumefunctions as a primary one or a secondary one of a pair.

The mating volume information 233 to 235 are the volume information onthe other party of a pair. In the case of a pair in the same storagecontrol unit, the secondary volume number is registered in column 233.For a pair between different storage control units, on the other hand, avolume number virtualized in the same storage control unit is registeredin column 233, the storage control unit number of the secondary volumehaving actually stored the data therein is registered in column 234, andthe volume number in column 235. The volume-occupied column 236 is theinformation indicating whether a particular volume is occupied orvacant.

Assume that FIG. 10 shows the volume information of the storage controlunit No. 0. The volume 0 produces three pairs. The first pair indicatesthat the volume 1024 of the storage control unit No. 0 is a virtualizedsecondary volume and the secondary volume having an actual data storedtherein is volume 20 of the storage control unit No. 1. The second pairindicates the volume 158 in the same storage control unit. The volume 1is used as a secondary volume of the pair, and the primary volume is thevolume 3783 of the storage control unit No. 3.

FIG. 11 is a diagram showing an example of the difference bit map 240.Two bit maps of the same size are prepared for one pair. In thedifference bit map, “0” indicates the point where replication isfinished, and “1” where the replication is yet to be finished. The dataof a predetermined size is rendered to correspond to one bit. In thecase where the data of 64 KB to correspond to one bit and even 1B of 64KB is updated, for example, the bit is set to “1” to reflect thecontents in the replication volume.

FIG. 14 is a diagram showing an example of the job priority informationbit map 250. This bit map indicates the information on that volumecontrolled by a given storage control unit having the bit map storedtherein which constitutes a replication pair with a volume controlled byanother storage control unit. The bit “1” indicates the desirability ofthe processing lower in priority than the normal read/write process. Thebit “0” indicates the desirability of the processing with a scheduleequivalent to that of the normal read/write processing. Assume that thebit map of FIG. 14 corresponds to the order of volume number. Theleading bit represents the information of volume 0. In this bit map, thefourth, fifth and eighth bits are “1”. This indicates that the volume 3,4 and 7 are processed normally when the read/write request is issued alesser number of times without affecting the normal read/writeprocessing ability. This bit map is indicated by “0” and “1”. In thecase where the scheduling is desired in several stages of priority,however, the number of bits per volume may be increased.

With this system configuration, an explanation is given about a methodof copying a volume of the storage control unit 20A to the storagecontrol unit 20B.

The process of producing a copy volume is executed according to thereplication creation program 201. The replication creation program 201checks to see whether the primary and secondary volumes of a replicationpair belong to the same storage control unit or to different storagecontrol units. In the case where the primary and secondary volumes areassociated with different storage control units, the process of thisinvention is executed. In the case where the primary and secondaryvolumes are associated with the same storage control unit, on the otherhand, the conventional process is executed.

FIG. 5 is a flowchart showing an outline of the process for creating areplication in the cluster configuration storage system of FIG. 1.First, a secondary volume providing a replication volume is selectedfrom vacant volumes, and a pair of primary and secondary volumes isregistered in the volume pair information table 220 (step 5010). It isdetermined whether the pair is associated with the same storage controlunit 20 (step 5020), and in the case where the pair is associated withthe same storage control unit 20, the process of producing a replicationin the same storage control unit is executed (step 5030) thereby toterminate the process. In the case where the volumes of the pair arelocated in different storage control units, on the other hand, thereplication creation process between different storage control units isexecuted (step 5040) thereby to terminate the process.

Next, the process of creating a replication in the same storage controlunit in step 5030 is explained with reference to the flowchart of FIG.6.

The initial copying process is started to copy all the contents of theprimary volume to the secondary volume. In the initial copying process,all the bits of the difference bit map 240 P1 shown in FIGS. 2 and 11are set to “1” (step 6010). Upon sequential detection of bits “1” on thedifference bit map (step 6020), it is determined whether the data at aposition corresponding to a particular bit “1” is stored in the cachememory or not (step 6030). Upon detection of no bit “1” in step 6020, onthe other hand, the process proceeds to step 6070. In the case where thecorresponding data is not stored in the cache memory in step 6030, thedata is read from the primary volume into the cache memory (step 6040).A copy of the primary volume is produced as a secondary volume data inthe cache memory (step 6050). At the same time as the copy operation,the redundant information to determine whether the data are correct ornot is newly produced for the secondary volume and attached to the data.Once the data are stored in the cache memory, the difference bit is setto “0” (step 6060). In the presence of the next bit, the process ofsteps 6020 to 6060 is repeated (step 6070). In the absence of the nextbit, on the other hand, the process is terminated. In an alternativemethod of reading the data from the primary volume, the redundantinformation for the secondary volume is produced and stored directly inthe cache memory as secondary volume data. On the other hand, thesecondary volume data on the cache memory are stored in the secondaryvolume (step 6080).

Next, the process of step 5040 for creating a replication betweendifferent storage control units is explained with reference to FIGS. 7and 8.

FIG. 7 shows an embodiment in which a replication is created betweendifferent storage control units. According to this embodiment, the datais desirably copied to the volume 312 from the volume 311 as an originalvolume. In the storage control unit 20A having the primary volume 311,the secondary volume 312 in the storage control unit 20B is virtualizedinto a volume 313 of the storage control unit 20A. As a result, the hostexecutes the replication creation process on the assumption that thevolumes of the pair (including the volumes 311 and 313 as pair No. 1)are located in the same storage control unit 20A. On the other hand, thestorage control unit 20A, which recognizes that the secondary volume isactually located in the storage control unit 20B, issues a write requestto transmit the copy data to the storage control unit 20B. The pairinformation of pair No. 1 (pair #1) is registered in the pairinformation table 220 in the storage control unit 20A having the primaryvolume. The pair information of pair No. 2 (pair #2), on the other hand,which is located in the storage control unit 20B, is registered in thepair information table 220 in the storage control unit 20B.

A replication, if any is created, is desirably created in the samestorage control unit and actually created in the same storage controlunit as far as possible. In the case where original volumes areconcentrated and no vacant area is available in a given storage controlunit, however, a replication may be created between a plurality ofstorage control units. An embodiment in which a replication is createdbetween different storage control units is shown in FIG. 7. In thisembodiment, a replication of the primary volume 311 is produced in areal secondary volume 312. Also in this case, a replication volume isselected in such a manner as to produce a replication of the primaryvolume 311 in a virtual secondary volume 313. In this way, thereplication appears to be created within the same storage control unit.The virtual secondary volume 313 uses no data area in the disk unit, butonly the volume number. This is the process for virtual data assignment.The host computer recognizes that the primary volume 311 is replicatedas a virtual secondary volume 313, and therefore can execute theprocess, as in the prior art, without recognizing the storage controlunit of the replication volume. The host computer issues a request tothe storage control unit having the primary volume 311 and the virtualsecondary volume 313. On the other hand, the storage system 70recognizes that the virtual secondary volume 313 is virtually assignedand that the actual volume is a real secondary volume 312. Once arequest arrives at the real secondary volume 312, therefore, it isreceived by the storage control unit of the virtual secondary volume 313and the contents of the request are transmitted to a storage controlunit having the real secondary volume 312.

FIG. 8 is an example of a flowchart showing the replication creationprocessing method between different storage control units. Thereplication creation processing method between different storage controlunits in step 5040 is explained below with reference to the flowchart ofFIG. 8.

Steps 8010 to 8070 are identical to steps 6010 to 6070. Though not shownin detail in FIG. 8, the process of the following steps is executed: Instep 8010 (corresponding to step 6010), all the bits of the differencebit map 240 P1 shown in FIGS. 2 and 11 are set to “1”. In step 8020(corresponding to step 6020), upon detection of bits “1” on thedifference bit map, step 8030 (corresponding to step 6030) determineswhether the data at a position corresponding to a particular bit “1”, iscontained in the cache memory or not. Unless bit “1” is detected in step8020, the process proceeds to step 8070 (corresponding to step 6070). Inthe case where the data at a position corresponding to bit “1” is notfound in the cache memory in step 8030, step 8040 (corresponding to step6040) reads the data from the primary volume into the cache memory. Instep 8050 (corresponding to step 6050), the data is copied into thesecondary volume in the virtualized cache memory. Once the data isstored in the cache memory, step 8060 (corresponding to step 6060) setsthe difference bit to “0”. In the presence of the next bit in step 8070(corresponding to step 6070), the process of steps 8020 to 8060 isrepeated. In the absence of the next bit, the process is terminated. Inthis way, the virtualized secondary volume data is stored on the cachememory. The process up to this stage is similar to that in the casewhere both primary and secondary volumes are located in the same storagecontrol unit.

On the other hand, the virtualized secondary volume data storedasynchronously on the cache memory is required to be stored in theactual secondary volume. For this purpose, a request to create areplication from the primary storage control unit 20A is transmitted tothe secondary storage control unit 20B and a replication is created inthe secondary storage control unit 20B. Specifically, a replication iscreated in a disk unit connected to the secondary storage control unit20B in the following manner: The storage control unit 20A having aprimary volume issues to the storage control unit 20B a request to writethe data (dirty data) on the cache memory not reflected in the storageunit of the storage control unit 20B for producing a secondary volume(step 8110). The secondary storage control unit 20B secures a cachememory for storing the write data (step 8120). The primary storagecontrol unit 20A is notified that the cache memory has been secured(step 8130). The primary storage control unit 20A, upon receipt of thenotification, transfers the write data (step 8140). The primary storagecontrol unit 20A receives a transfer completion notification from thesecondary storage control unit 20B (step 8150). The primary storagecontrol unit 20A, in the presence of the next dirty data, repeats theprocess of steps 8110 to 8150 (step 8160). In the absence of the nextdirty data, on the other hand, the process is terminated. The secondarystorage control unit 20B stores the data from the cache memory into thesecondary volume asynchronously (step 8170).

The initial copy process is for copying the data corresponding to allthe difference bits. Even after complete initial copy process, thedifference bit map is set to “1” if a write request is received midway.In this residual copy process, the difference bit map is searched fromthe head sequentially, and upon detection of a bit “1”, a similar copyprocess as the initial copy process is executed.

The normal read/write request may arrive during the initial copyprocess. The process to be executed when a write request arrives isexplained below with reference to the flowchart of FIG. 9.

The storage control unit 20A receives a write request from the host 10(step 9010). The storage control unit 20A sets to “1” the bit at aparticular position of the difference bit map 240 P1 (FIGS. 2 and 11)corresponding to the data to be written (step 9020). The storage controlunit 20A secures the cache memory area for storing the write data (step9030). The storage control unit 20A receives the write data from thehost 10 and stores it in the cache memory (step 9040). The storagecontrol unit 20A returns a write completion notification to the host 10(step 9050).

The storage control unit 20A stores the write data independence of thewrite request from the host in the primary volume asynchronously (step9060).

The process of reflecting the contents of the write data in thesecondary volume is executed in such a manner that the difference bitmap is sequentially watched, and upon detection of a bit “1” (step 9070)with a pair in the same storage control unit (step 9080), the process ofsteps 6030 to 6060, 6080 is executed (step 9090), while the process ofsteps 8030 to 8060, 8110 to 8150, 8170 is executed for a pair betweendifferent storage control units (step 9100). In search of the next bit,the process proceeds to step 9070, and in the absence of the next bit,the process is terminated (step 9110).

Next, the split state of the pair is explained. Once a pair is split,the difference bit maps 240 P1 and 240 P2 are switched. As shown in FIG.11, the difference bit maps P1 and P1 are held in the memory 22 for eachpair in the memory 22 of the storage control unit of the primary volume.The difference bit maps P1 and P2 are assumed to have the same size. Thedifference bit maps used during the split state are held in the memory22 of the storage control unit of the primary volume, and each time thesecondary volume is updated, the difference bit maps in the storagecontrol unit 20A are accessed.

For resynchronization with the contents of the primary volume, theprevailing contents of the primary volume are copied to the secondaryvolume. For this purpose, the two difference bit maps, primary andsecondary, are merged and the same process is executed as the initialcopy process. In other words, in the case where one of the bits forprimary or secondary volume is set to “1”, all the volumes are copied.

The normal split process is executed at the time point when the contentsof the primary and secondary volumes are synchronized with each otherafter initial copy process. Apart from this, the “fast split” process isexecuted in such a manner that upon receipt of a split request evenduring the initial copy process, a split completion is notified to thehost 10 while the reprimarying copy process is executed in thebackground. FIG. 12 is a flowchart showing the split process accordingto this method. As shown in FIG. 12, the host 10 issues a “fast split”instruction, and the storage control unit 20A, upon receipt of theinstruction (step 12010), switches the difference bit map 240 forstoring the write request of the host 10 from P1 to P2 (step 12020). Thestorage control unit 20A changes the pair status 226 (FIG. 4) of thevolume pair information table 220 to “split” (step 12030). Now, theprimary and secondary volumes are ready to receive a read/write request.The storage control unit 20 executes the process of copying theunreflected data to the secondary volume in the background. The processof reflecting the data in the secondary volume is executed in such amanner that the difference bit map is sequentially watched, and upondetection of a bit “1” (step 12040) with a pair in the same storagecontrol unit 20 (step 12050), the process similar to steps 6020 to 6080is executed (step 12060). In the case where a pair between differentstorage control units 20 is involved, on the other hand, the process ofsteps 8020 to 8070 and steps 8110 to 8160 is executed (step 12070). Insearch of the next bit, the process proceeds to step 12040. In theabsence of the next bit, the process is terminated (step 12080).

The process executed when a write request is received by the primaryvolume from the host 10 in fast split mode is shown in FIG. 13.

The storage control unit 20A receives a write request into the primaryvolume from the host 10 (step 13010). The storage control unit 20A setsthe bit to “1” at a position corresponding to the write data in thedifference bit map 240 P1 (FIG. 11) (step 13020). It is determinedwhether the data at the position corresponding to the particular bit isin the cache memory or not (step 13030). In the case where the data isnot found in the cache memory in step 13030, the corresponding data isread into the cache memory from the primary volume (step 13040). Upondetection bit “1” at a position corresponding to the old write data ofthe difference bit map P2, the process proceeds to step 13060.Otherwise, the process proceeds to step 13080 (step 13050). In the casewhere a bit is set to “1”, it indicates that the data corresponding tothe particular bit has yet to be copied to the secondary volume. The olddata is copied to the data area of the secondary volume in the cachememory (or the secondary volume area virtualized in the cache memory inthe case of the pair between different storage control units) (13060).In the absence of the data in the cache memory, on the other hand, thedata may be read twice for the primary and secondary volumes. With thecopy operation, the redundant information for determining whether thedata is correct or not is newly produced for the secondary volume andattached to the data. Once the data is stored in the cache memory, thedifference bit is set to “0” (step 13070). As the result of executingthe process of steps 13030 to 13070 described above, the old data of theprimary volume is stored in the secondary volume data area in the cachememory (or the secondary volume data area virtualized in the cachememory in the case of a pair between different storage control units).After step 13070, or in the case where determination in step 13050 isNO, the data to be written to the bit involved is received from the host10, and stored in the cache memory area for the primary volume (step13080). A write completion notice is sent to the host 10 (step 13090).

In the case of a pair associated with the same storage control unit(step 13100), the data on the cache memory are stored in the secondaryvolume asynchronously (step 13110). In the case of a pair betweendifferent storage control units, on the other hand, in order to storethe old data on the cache memory in the storage control unit 20B havinga secondary volume having stored therein the actual data, the process ofsteps 8110 to 8160 is executed (step 13120).

Next, an explanation is given about a case in which the secondary volumereceives a write request from the host 10 in fast split mode. Referenceis had to the flowchart of FIG. 13.

Like in step 13010, the storage control unit 20A receives from the host10 a write request to the virtualized secondary volume. Like in step13020, the storage control unit 20A sets the bit to “1” at a pointcorresponding to the write data in the difference bit map 240 P1 (FIG.11). Like in step 13030, it is determined whether the data at a pointcorresponding to the bit is a cache memory area for the primary volume.Like in step 13040, in the case where the data is not available in thecache memory in step 13030, the corresponding data is read from theprimary volume into the cache memory. Like in step 13050, upon detectionof a bit “1” at a point of the difference bit map 240 P2 correspondingto the old write data, the process proceeds to step 13060. Otherwise,the process proceeds to step 13080. Like in step 13060, the old data iscopied to the data area of the secondary volume in the cache memory (orthe secondary volume area virtualized in the cache memory in the case ofa pair between different storage control units). In the absence of datain the cache memory, the data may be read twice for the primary andsecondary volumes. In the case where the data is read from the cachememory, the old data may be read directly into the data area of thesecondary volume in the cache memory (or the secondary volume areavirtualized in the cache memory in the case of a pair between differentstorage control units). By doing so, only one read process is enough.With the copy operation, the redundant information to determine whetherthe data is correct or not is also newly produced for the secondaryvolume, and with the data, stored in the cache memory. In the case wherethe write data are coincident with all the data corresponding to thebits, the old data is not required to be stored in the cache memory, andtherefore only the area is secured. Once the data is stored in the cachememory, the difference bit is set to “0” (like in step 13070). The datato be written to the bit involved is received, and stored in the cachememory area for the secondary volume (like in step 13080). A writecompletion notice is issued to the host 10 (like in step 13090). Afterthat, the same process is executed as steps 13100 to 13120. The hostissuing a write request to the secondary volume may be other than thehost 10 issuing a write request to the primary volume.

Next, the resynchronization of the volumes is explained with referenceto the flowchart of FIG. 15. The primary volume and the secondary volumethat have entered the split state are processing read and writerequests, respectively, and therefore have different contents. Theresynchronization is defined by the process of setting the contents ofthe secondary volume to the contents of the primary volume at theparticular time point. First, the pair status of the volume pairinformation is changed (step 15010). The difference bit maps 240 P1 and240 P2 are merged with each other and stored in the difference bit map240 P1 (step 15020). Upon detection of bit “1” in the difference bit map240 P1 (step 15030), the process similar to the initial copy process isexecuted. Specifically, for a pair in the same storage control unit(step 15040), the process of steps 6020 to 6080 is executed (step15050), while the process of steps 8020 to 8160 is executed for a pabetween different storage control units (step 15060). In search of thenext bit, the process proceeds to step 15030. Otherwise, the process isterminated (step 15070).

As explained above, in the case where the original volume and thereplication volume are associated with different clusters, the pair maybe rendered to appear to belong to the same cluster to the host by thevirtualization technique in which the actual replication volume isvirtually assigned to a vacant volume in the cluster of the originalvolume. As a result, a cluster configuration storage system is providedfor producing a copy of the storage area freely without being consciousof the cluster.

The replication creation process is explained above. The processingspeed priority process is explained below. In the processing speedpriority process, the process of producing a replication volume and thenormal read/write request (normal I/O) are processed in the order ofpriority according to the prevailing situation.

In the replication process described above, the process due to the copyprocess is transferred from the primary cluster to the secondary clusterregardless of the situation of the secondary cluster. Nevertheless, thesecondary cluster receives the normal host I/O. Also, the secondarycluster has an copy function engine and produces a copy of the volume inthe cluster. In this way, each cluster has the copy function and a jobrequest queue, so that jobs are started and processed in accordance withthe order of priority of the job types.

In the case where the replication process is executed over clusters andthe communication is conducted over the clusters in the normalread/write request fashion, the party that has received the requestcannot distinguish it from the normal I/O from the host. Nevertheless,the host I/O is desirably executed in priority over the process of thecopy function.

Specifically, in the case of a pair between different storage controlunits, the storage control unit 20B having a secondary volume has no wayof distinguishing the write request received from the primary storagecontrol unit 20A from the normal write request received from the host10. Thus, the storage control unit 20B processes all the write requestson first-come first-served basis. As a result, the response time of thenormal write request is lengthened, and the performance sometimesappears to be deteriorated. These write requests are discriminated bythe storage control unit 20B and a schedule is formed for the order inwhich they are executed, thereby realizing a high performance.

The embodiment described below refers to the processing speed priorityprocess in which the original volume sets the job priority in thereplication volume.

Assume that the write request received from the primary storage controlunit 20A and the normal write request received from the host 10 arediscriminated from each other by volume unit. In the case of a volume inthe storage control unit 20B of which a replication is being created,the storage control unit 20A recognizes whether the particular volume isin paired state or not. The storage control unit 20A can thus determine,by the pair status, whether the request to the volume creating areplication in the storage control unit 20B is for the copy process tocreate a replication or for the normal read/write operation.

This is explained with reference to the flowchart of FIG. 16. Accordingto the job priority set program 202 (FIG. 2) in the storage control unit20A, the job priority is determined as to whether a request to thevolume in the storage control unit 20B paired with the volume in thestorage control unit 20A is to be processed in the same manner as thenormal read/write request (step 16010). The job priority informationdetermined in the storage control unit 20A is delivered to the storagecontrol unit 20B (step 16020). The storage control unit 20B registersthe job priority information at the bit position of the volume number inthe job priority information bit map 250 (FIGS. 2 and 14) in the memory22 (step 16030). The job priority information bit map 250 manages theinformation as to whether the data is to be processed according to aschedule for each volume number like the normal read/write request.

In the case where it is desired to execute the process rapidly and aschedule is formed without discrimination from the normal read/writerequest, the bit is set to “1”, while the bit is set to “0” when thenormal read/write request is desirably processed in priority. As analternative, the job priority of the normal read/write request is set to5, and the other requests may be managed in the order of priority of 4to 1. In the process, several bits, but not one bit, are prepared foreach volume.

During the creation of a replication, assume that a read/write requestis issued in step 8110 by the storage control unit 20A to the storagecontrol unit 20B (step 16040). The storage control unit 20B receives therequest, and accesses the job priority information of the volume for therequest (step 16050). As far as the job priority information is adaptedto be scheduled like the normal read/write request (step 16060), it isplaced in the normal read/write processing queue 261 (FIG. 2) (step16070), and when the request is selected from the queue, the cachememory area is secured in step 8120 and the steps 8130 to 8150 areexecuted (step 16080). In the case where the job priority information islower in priority than the normal read/write request in step 16060, thedata is placed in another low job priority queue 262 (FIG. 2) than thenormal read/write processing queue 261 (step 16090), and when the datais selected, the process of step 16080 is executed. The priority orderof the job priority information of the volume may be input by aprimarytenance worker from, for example, the user input/output apparatus80 such as SVP (SerVice Processor).

The normal read/write processing queue 261 and the low job priorityqueue 262 are the control information having a queue structure managedin the order of the time of issuance in accordance with the arrangementor the list structure.

The real secondary volume 312 corresponding to the virtualized volume313 (FIG. 7) is transparent to the host 10, and therefore no request isissued to the volume 312 from the host 10. The request to the volume 312arrives only through the storage control unit 20A. Thus, the storagecontrol unit 20A can determine whether the request of the volume 312 isto be processed immediately or not.

The scheduling for the low job priority queue 26 is explained withreference to the flowchart of FIG. 17. The low job priority queue 262 islower in processing frequency than the normal read/write processingqueue 261. The processing frequency α is changeable by the settingoperation of the user.

Assume that the processing frequency is α. It is determined whether thenormal read/write queue 261 has an unprocessed request or not (step17010). If there is any unprocessed request, the counter c is set to 0(step 17020). In the case where α>c (step 17030), the process of thenormal read/write processing queue 261 is executed (step 17040), and thecounter c is incremented (step 17050). It is determined whether thenormal read/write processing queue 261 has a request yet to be processed(step 17060). In the presence of such a request, the process proceeds tostep 17030. In the absence of such a request, on the other hand, theprocess proceeds to step 17100. Once the relation α≦c is achieved instep 17030, it is determined whether the number of unprocessed requestsof the low job priority queue 262 is not smaller than n, or whether theold unprocessed request of the low job priority queue 262 has passed thetime m (step 17070), where m and n are predetermined numerical values.Once the condition of step 17070 is met, the process of the low jobpriority queue 262 is executed (step 17080). The counter c is set to 0,and the process proceeds to step 17060 (step 17090). Unless thecondition is met in step 17070, on the other hand, the process proceedsto step 17090.

In the case where it is determined in step 17010 that there is norequest yet to be processed, the process of the low job priority queue262 is executed, and the process proceeds to step 17010 (step 17110).

In the case where a read/write request is issued from the storagecontrol unit 20A to the storage control unit 20B in step 16020, thestorage control unit 20A issues the request without being conscious ofthe state of the storage control unit 20B. The amount of the dirty datain the cache memory of the storage control unit 20B may be increased bythe normal read/write process or the replication process in the storagecontrol unit 20B. The increase in the dirty data amount beyond apredetermined reference value indicates a smaller available capacity ofthe cache memory. By suspending the replication process or executing theprocess to secure the data from the cache memory to the disk unit inpriority, therefore, the vacant area of the cache memory is required tobe increased. The storage control unit 20B accesses the job priorityorder of the volume for the request in step 16050, while at the sametime accessing the dirty data amount. In the case where the dirty dataamount is increased beyond a predetermined (preset) reference value, theprocess of steps 8120 to 8150 for securing the cache memory area anddelaying the process of receiving the data is executed thereby to adjustthe amount of the requests to be received from the storage control unit20A. The storage control unit 20A stops issuing a request beforereceiving a completion notice from the storage control unit 20B.

Next, a modification of the processing speed priority process isexplained for supplying information from the primary cluster to thesecondary cluster as to whether the replication volume is processedwithout affecting the normal read/write request (normal I/O) orequivalently to the normal read/write request (normal I/O). Unlike inthe embodiment described above, the modification described belowrepresents a case of the processing speed priority process in which thereplication volume determines whether a process is to be executed inpriority or nonpriority.

In the case where the request transmitted from the primary cluster tothe secondary cluster is a copy processing request (request to create areplication), the primary cluster (original volume) transmits therequest to the secondary cluster together with a request type indicatingthat the particular request is a copy processing request (request tocreate a replication). The secondary cluster (replication volume) thathas received the request recognizes that the request with the requesttype attached thereto is low in priority, and places the request in thelow job priority queue 262 (FIG. 2). In this way, the secondary clustercan determine the job priority based on the request type received fromthe primary cluster. In the case under consideration, the primarycluster transmits a request to create a replication with the informationindicating that the particular request is to create a replication.Nevertheless, it is always the secondary cluster that determines whetherthe request is low in priority or not. Thus, the secondary cluster candetermine whether the request to create a replication is to be processedas a low priority.

In another method, the primary cluster (original volume) adds to theinstruction general information indicating whether a request be givenpriority or not or the priority n of the request, but not a specificrequest type indicating a copy request. Upon receipt of thisinstruction, the secondary cluster (replication volume) recognizes thepriority information thus added and forms a schedule of the processingorder taking the particular priority into consideration. As a result,the secondary cluster can determine the job priority based on thepriority order n or the priority/non-priority information of the requestreceived from the primary cluster. Also in this case, it is thesecondary cluster that determines whether a request to create areplication is to be processed as a low priority. In this case, however,the determination of the secondary cluster is based on the priorityorder n or the ordinary priority/non-priority information included inthe request. Thus, the secondary cluster can determine whether a requestto create a replication is to be processed in priority by the normalrequest processing.

The primary cluster gives the secondary cluster information as towhether the replication volume is processed without affecting the normalI/O or equivalently to the normal I/O. In the former case, the secondarycluster, by reference to this information. In the latter case, on theother hand, a schedule is formed to process the requests inchronological order.

The processing speed priority process described above makes possible areplication without interfering with the normal business I/O.

A cluster configuration storage system according to an embodiment of theinvention was explained above. With reference to FIG. 18, an explanationis given about the user operation for realizing a pair in the samecluster from a pair between different clusters by the virtualizationtechnique of the cluster configuration storage system described above.

First, the user selects a replication volume. Vacant volumes 321, 322,323, 324 constituting candidates for a replication volume against theoriginal volume 311 are presented as a secondary volume pool to theuser. The secondary volume pool includes a volume of a cluster differentfrom the original volume.

The user selects a secondary volume by inputting the vacant volume 322,for example, from the user input/output apparatus 80. In the case shownin FIG. 18, the selected volume 322 is associated with a cluster(storage control unit 20B) different from the cluster (storage controlunit 20A) of the original volume 311.

The replication volume is associated with a different cluster, andtherefore a secondary volume to be virtualized is selected from thevacant volumes of the original cluster. In the case under consideration,the volume 312 is selected as a virtual secondary volume. This volume isnot a real volume, and therefore requires no physical resource such asHDD. Device numbers exclusively used for virtualization may be preparedand an appropriate one of them may be selected.

In this case, the secondary volume candidate is existent in the samecluster as the original volume, and therefore the particular volume maybe presented from the system to the user so that the user may be allowedto select the same volume from among the candidates. As anotheralternative, the user is presented with volume candidates and a volumeis selected from the secondary volume pool and/or from virtualizedvolumes, by input from the user input/output apparatus 80, or a hostcommand is used or a candidate is automatically selected on the part ofa storage control unit.

The replication creation program 201 (FIG. 2), as described above,registers the information in the volume pair information table 220 basedon the information from the user input/output apparatus or the hostcommand from the host. As another alternative, the replication creationprogram 201 registers the information automatically in the volume pairinformation table 220.

The invention developed by the present inventor has been specificallydescribed above with reference to embodiments. This invention, however,is not limited to the embodiments described above, but variouslymodifiable without departing from the spirit and scope of the invention.

1.-20. (canceled)
 21. A storage system coupled to a host computercomprising: a plurality of control units each connected with arespective disk unit and controlled to read/write data sent from thehost computer from/to the respective disk unit, wherein the plurality ofcontrol units form a pair relationship between a first volume and asecond volume in the storage system, the plurality of control unitscontrol to copy between the first volume and the second volume, and thesecond volume is a virtual volume related to a logical volume.
 22. Astorage system according to claim 21, wherein each of the control unitsincludes a cache memory for temporarily storing the data, and areplication creation unit being adapted to create a copy of the data ofa logical volume relating to the disk unit connected with thecorresponding control unit; and wherein the replication creation unit ofa first control unit, in creating a copy in the volume of the disk unitconnected to a second control unit, copies the data of the first volumeas a data for the replication volume in the cache memory of the firstcontrol unit, and transmits the copy data in the cache memory to thesecond control unit.
 23. A storage system according to claim 22, whereinthe second control unit stores the data received from the first controlunit in the cache memory of the second control unit, after which thedata is stored in the volume relating to the disk unit connected to thesecond control unit.
 24. A storage system according to claim 21, whereineach of the control units includes a replication creation unit beingadapted to create a copy of the data of a logical volume relating to thedisk unit connected with the corresponding control unit; wherein thevolume information of the replication volume of a first control unit inthe pair relationship in the case where a copy is created in the volumeof the disk unit connected to a second control unit, is virtual IDinformation for identifying, in the first control unit, the volume ofthe disk unit connected to the second control unit; and wherein theinformation on the second control unit in the pair relationship in thecase where a copy is created in the volume of the disk unit connected tothe second control unit is ID information for identifying, in the secondcontrol unit, the second control unit and the replication volume of thedisk unit connected to the second control unit.
 25. A storage systemaccording to claim 21, wherein the pair relationship includes anidentifier assigned to the volume pair of the first volume and thesecond volume, and wherein one or a plurality of the identifiers areassigned to the first volume.
 26. A storage system according to claim21, wherein each of the control units includes a replication creationunit being adapted to create a copy of the data of a logical volumerelating to the disk unit connected with the corresponding control unit;and wherein the replication creation unit forms information in the pairrelationship based on selected one of the information input from a userinput/output apparatus and a host command received from a host.
 27. Astorage system according to claim 21, wherein each of the control unitsfurther includes a plurality of normal read/write processing queuescorresponding to the plurality of the control units, respectively, eachof the queues being adapted to form a schedule for a normal read/writerequest, and a plurality of low job priority queues corresponding to theplurality of the control units, respectively, each of the low jobpriority queues being adapted to form a schedule for a request low injob priority than the normal read/write request; wherein in the casewhere a request for creating a copy is processed equivalently to thenormal read/write request, the control unit places a request forcreating a copy in the normal read/write processing queue, while in thecase where a request for creating a copy is processed in the order ofpriority lower than the normal read/write request, the control unitplaces the copy creation request in the low job priority queue.
 28. Astorage system according to claim 21, wherein each of the control unitsfurther includes a plurality of job priority set units corresponding tothe plurality of the control units, respectively, each of the jobpriority set units determining the job priority information as towhether a request for creating a copy is to be processed equivalently tothe normal read/write request, the job priority set unit notifying thejob priority order information to the other control units, and wherein acontrol unit that has received the notification of the job priorityinformation from other control units forms a schedule for sequentiallyprocessing the requests based on the job priority information thusreceived.
 29. A storage system according to claim 21, wherein in thecase where a first control unit transmits a request for creating a copyto a second control unit, the information indicating a copy creatingprocess is added to the control instruction for transmitting the copycreation request, and wherein the second control unit that has receivedthe control instruction determines whether the request is to beprocessed in priority, based on the information indicating a copycreation process, and forms a schedule for sequentially processing therequests.
 30. A storage system according to claim 21, wherein in thecase where a first control unit transmits a request for creating a copyto a second control unit, the information indicating the priority orderis added to the copy creation request, and wherein the second controlunit determines as to whether a copy creation request is processed inpriority based on the received information indicating the priority, andforms a schedule for sequentially processing the copy creation requests.31. A method of creating a copy in a storage system coupled to a hostcomputer, the storage system including a plurality of control units eachconnected with a respective disk unit and controlled to read/write datasent from the host computer from/to the respective disk unit, comprisingthe steps of: forming a pair relationship between a first volume and asecond volume in the storage system, and controlling to copy between thefirst volume and the second volume, wherein the second volume is avirtual volume related to a logical volume, wherein the forming andcontrolling steps are performed by the plurality of control units; andwherein, in the case where the replication is created in the logicalvolume relating to the disk unit connected to the second control unit, areplication data of the original logical volume is transferred from thefirst control unit to the at least one logical volume of the secondlogical unit based on the volume pair information.
 32. A replicationcreation method according to claim 31, wherein each of the control unitshas a cache memory for storing the data temporarily, the method furthercomprising the steps of copying in a first control unit the data of thefirst volume to the cache memory of the first control unit, and sendingfrom the first control unit the data stored in the cache memory to asecond control unit.
 33. A replication creation method according toclaim 32, further comprising the steps of: storing, in the secondcontrol unit, the data received from the first control unit into thecache memory of the second control unit, and storing, in the secondcontrol unit, the data in the cache memory of the second control unitinto the volume relating to the disk unit connected to the secondcontrol unit.
 34. A replication creation method according to claim 31,wherein in the case where a copy is created in the volume of the diskunit connected to a second control unit, the volume information of thereplication volume of a first control unit in the pair relationship isvirtual ID information for identifying, in the first control unit, thevolume of the disk unit connected to the second control unit, andwherein in the case where a copy is created in the volume of the diskunit connected to the second control unit, the information on the secondcontrol unit in the pair relationship is the information for identifyingthe second control unit and the information for identifying, in thesecond control unit, the replication volume relating to the disk unitconnected to the second control unit.
 35. A replication creation methodaccording to claim 31, wherein the pair relationship includes anidentifier assigned to the pair of the first volume and the secondvolume, and wherein one or a plurality of identifiers are assigned tothe first volume.
 36. A replication creation method according to claim31, wherein a control unit forms the information in the pairrelationship based on a selected one of the information input from auser input/output apparatus and a host command received from the host.37. A replication creation method according to claim 31, wherein each ofthe control units includes a normal read/write processing queue toschedule the normal read/write request, and a low job priority queue toschedule the request lower in priority than the normal read/writerequest, and wherein in the case where a copy creation request isprocessed equivalently to the normal read/write request, the controlunit places the copy creation request in the normal read/writeprocessing queue, while in the case where a copy creation request isprocessed in a priority lower than the normal read/write request, thecopy creation request is placed in the low job priority queue.
 38. Areplication creation method according to claim 31, wherein the storagesystem has a job priority set unit for each of the control units,wherein the job priority set unit of a first control unit sets the jobpriority as to whether a copy creation request is to be processedequivalently to a normal read/write request, and notifies the jobpriority information to a second control unit, and wherein the secondcontrol unit forms a schedule for sequentially processing the copycreation requests based on the job priority information thus received.39. A replication creation method according to claim 31, wherein a firstcontrol unit transmits a copy creation request to a second control unitwith the information indicating a copy creating process, and wherein thesecond control unit determines whether the received copy creationrequest is to be processed in priority or not, based on the informationindicating a copy creation process, and forms a schedule forsequentially processing the copy creation requests.
 40. A replicationcreation method according to claim 31, wherein a first control unittransmits a copy creation request to a second control unit with theinformation indicating the order of priority, and wherein the secondcontrol unit determines whether the copy creation request is to beprocessed in priority or not, based on the information indicating theorder of priority, and forms a schedule for sequentially processing thecopy creation requests.
 41. A storage system according to claim 21,further comprising a replication creation unit being adapted to create acopy of the data of a logical volume relating to the disk unit connectedwith the corresponding control unit; and wherein the replicationcreation unit of a first control unit operates in such a manner that inthe case where a copy of a first volume, relating to the disk unitconnected to a first control unit, is created in the logical volumerelating to the disk unit connected to the first control unit, thevolume information of the first volume and the volume information of thecopy volume are formed in the pair relationship, and the copy is createdin the logical volume relating to the disk unit connected to the firstcontrol unit based on the pair relationship, and in the case where acopy of a first volume, relating to the disk unit connected to the firstcontrol unit, is created in a logical volume relating to the disk unitconnected to a second control unit, the volume information of the firstvolume, virtual volume information of a virtual replication volumevirtualized in the disk unit connected to the first control unit, andinformation on a second control unit are registered in the pairrelationship, and a request to create a copy is transmitted to thesecond control unit based on the pair relationship when a copy requestis sent from the host computer for creating the replication volume inthe disk unit connected to the first control unit; wherein in the casewhere a copy is created in the volume relating to the disk unitconnected to the second control unit, the replication creation unit ofthe first control unit issues to the second control unit a request towrite the data corresponding to the virtual replication volume to the atleast one logical volume of the second control unit corresponding to thevirtual replication volume.
 42. A replication creation method accordingto claim 31, further comprising the steps of: registering in a firstcontrol unit first volume information and second volume information asthe pair relationship, in the case where a copy of the data of the firstvolume in a logical volume relating to the disk unit connected to thefirst control unit is created in the logical volume relating to the diskunit connected to the first control unit; registering in the firstcontrol unit the first volume information, virtual volume information ofa virtual replication volume virtualized in the disk unit connected tothe first control unit, and information on a second control unit in thepair relationship of the first control unit, in the case where a copy ofthe data of an original volume, relating to the disk unit connected tothe first control unit, is created in a logical volume relating to thedisk unit connected to the second control unit, wherein the virtualvolume information of the virtual replication volume includes a volumenumber of a volume virtualized in the disk unit connected to the firstcontrol unit but for which no volume is created in the disk unitconnected to the first control unit; generating in the first controlunit a copy of the first volume relating to the disk unit connected tothe first control unit, based on the pair relationship, in the casewhere a copy of the data of the first volume relating to the disk unitconnected to the first control unit is created in the logical volumerelating to the disk unit connected to the first control unit; andsending from the first control unit a copy creation request to thesecond control unit based on the pair relationship when a copy requestis sent from the host computer for creating the replication volume inthe disk unit connected to the first control unit, in the case where acopy of the data of the first volume in the logical volume relating tothe disk unit connected to the first control unit is created in thelogical volume relating to the disk unit connected to the second controlunit, wherein the host computer recognizes the replication volumecreated in the logical volume relating to the disk unit connected to thesecond control unit as having been created in the logical volumerelating to the disk unit connected to the first control unit; whereinthe step of sending a copy creation request from the first control unitto the second control unit includes sending a request to write data fromthe virtual replication volume in the first control unit to a logicalvolume corresponding thereto in the second control unit.